In the spring and early summer of 1997, stargazers were treated to a
beautiful sky show with the passing of wispy-tailed Comet Hale-Bopp. But
scientists now say that while all eyes were on this dazzling sight, another
near-Earth comet slipped quietly through the sky unnoticed.

Astronomers analyzing archived data from the European Space Agency/NASA
Solar and Heliospheric Observatory (SOHO) spacecraft have recently reported
the presence of a never-before-detected comet, which flew close to Earth in
1997. The comet, temporarily dubbed C/1997 K2, was apparently brighter than
every comet discovered by astronomers in the six months preceding its
appearance, adding to the mystery of how such a prominent comet may have
zoomed past Earth unseen.

"To say we were surprised would be a bit of an understatement," said Finnish
astronomer Teemu Mäkinen, lead author of a paper in this week's Nature,
which describes the new comet discovery. "It sounded quite unlikely [to us]
that a comet of such magnitude could elude both professionals and amateurs
alike."

Despite its brightness, Comet K2 would not have been visible to the naked
eye. Yet even "inexpensive amateur equipment would have sufficed" for
stargazers to see the comet, said Mäkinen. "I believe that many amateurs
were lured by the spectacular display of the concurrent Comet Hale-Bopp," he
explained.

Eluding the pros

Professional astronomers could have easily spotted the comet, since the
equipment they use is much more powerful than the average telescope. So
here's the question: how did they miss it? The combination of vast expanses
of space and limited resources with which to scan them are the most likely
reasons, experts say.

"Our records are patchy, especially at high latitudes," said Mäkinen. "It
was probably missed by dedicated surveys because of its trajectory, which
went through the southern ecliptic pole."

Though it slipped by unnoticed by human eyes, evidence for the K2 comet was
captured by SOHO's Solar Wind Anisotropies (SWAN) instrument from May to
July of 1997.

The SWAN instrument is not dedicated to comet discovery. Rather, it was
designed to observe emissions of hydrogen around the sun at ultraviolet
wavelengths. But these emissions, known as Lyman-a emissions, are also given
off in high quantities by comets. As a result, though its resolution is
relatively poor, the instrument is also sensitive to comets.

Could we miss the big one?

The discovery of Comet K 2 raises an interesting question: How prepared are
we to track near-Earth objects, including comets and asteroids. If we missed
this comet, some experts ask, what are the chances that we'll miss a comet
that's heading straight for Earth?

A great deal of work has taken place to locate and categorize near-Earth
objects like comets and asteroids. Although it may sound like science
fiction, scientists say that the threat of comet impacts on Earth is very
real.

To highlight the threat of such an impact, Massachusetts Institute of
Technology professor Richard P. Binzel even created a scale to measure the
probability that a given object will hit Earth and cause it harm. Called the
Torino scale, this method of measurement highlights the fact that Earth's
current environmental balance is precarious, and it and the life that
depends on it could be devastated in one swift blow.

So what are the chances of a comet hitting Earth, or of astronomers missing
a potentially dangerous on-coming object. No one really knows. What
astronomers do know, however, is that better funding for continual sky scans
would help them map all comets that streak through the solar system.

"It is just a matter of prioritization. Do we want to use limited resources
on preventing something that is likely to happen every day and cause
moderate casualties -- like traffic accidents -- or something that will
happen once in 30 million years but has potential for wiping out all of
humankind?" asked Mäkinen. "In such situations, unfortunately, myopia is
usually bliss."